Small low-density lipoprotein (LDL) particles are more atherogenic than larger LDL particles. To help prevent atherosclerotic coronary heart diseases, it may be useful to understand risk factors during childhood. In the present study, we evaluated LDL size and its relationship to other risk factors for atherosclerotic coronary heart disease. LDL size was measured by 2-15% gradient gel electrophoresis in 586 Japanese children (316 boys and 270 girls). Plasma lipids, apolipoproteins (apo), glucose, and insulin were also determined by conventional methods.Pattern B (LDL size < 25.5 nm) was found in 10.8% of boys and 4.4% of girls. Children with pattern B had a higher body mass index (BMI) and insulin resistance and a more atherogenic lipoprotein profile [higher triglycerides, higher apoB, and lower high-density lipoprotein cholesterol (HDL-C)] than children with pattern A (LDL size > 25.5 nm). BMI, insulin resistance, and plasma concentrations of triglycerides, glucose, and insulin decreased and plasma concentrations of HDL-C and apoA-I increased as LDL size increased. HDL-C and insulin in boys, and BMI, HDL-C, and apoA-I in girls predicted 22.9 and 28.1% of the variability of LDL size, respectively.LDL size was correlated with BMI and plasma concentrations of HDL-C, apoA-I, and insulin. Although the contribution of these parameters to LDL size in children was less than that in adults, improvement of these parameters by changes in lifestyle might be important for preventing the development of atherosclerosis even in children. (J Clin Endocrinol Metab 89: 2923-2927, 2004) P LASMA LEVELS OF low-density lipoprotein (LDL) cholesterol (LDL-C) are positively associated with the risk for coronary heart disease (CHD) (1, 2). However, several studies have shown that 30 -40% of CHD patients have normal LDL-C levels (Ͻ 130 mg/dl) (3-5). LDL particles vary in size and hydration density (6, 7). Several LDL subfractions can be identified on gradient gel electrophoresis. Among these LDL subfractions, small LDL particles possess a lower binding affinity for cellular LDL receptor and are more easily oxidized in vitro (8 -11). These data suggest that small LDL particles are atherogenic because their lower binding affinity for LDL receptor reflects a longer plasma residence time for them to be oxidized and taken up by macrophages in extravascular spaces. In accordance with these in vitro data, several case-control studies have indicated that smaller LDL particles are predominant in a high proportion of CHD patients, even those with normal LDL-C levels (12-15). Subjects with small, dense LDL have a 3-fold greater CHD risk (16). LDL particle size is associated with plasma triglyceride (TG), high-density lipoprotein (HDL) cholesterol (HDL-C), and fractional esterification rate in HDL (17, 18). In addition, age, sex, body fat, insulin resistance, and environmental (diet etc.) and genetic factors have been reported to affect LDL particle size (14, 19 -21).As previously mentioned, the relationships between LDL size and other risk factors f...